1. Field of the Invention
The present invention is directed to a debinding process of heating a powder compact shaped from a mixture of a sinterable powder and an organic binder for removing an excess amount of the organic binder from within the powder compact prior to sintering.
2. Description of the prior art
In the manufacture of a powder product, it is a general practice to provide a powder compact by compacting a sinterable powder and subsequently sintering the compact into the finished powder product. An organic binder is normally added to the sinterable powder to impart fluidity to the resulting mixture for facilitating the of the powder into a desired shape by press, extrusion, injection molding or other various forming methods. Among them, injection molding is mostly preferred to form the powder product into complicated shapes. However, injection molding requires high fluidity and therefore a greater amount of the organic binder to be mixed with the sinterable powder. After forming the powder product, an excess amount of the organic binder should be removed therefrom by heat treatment prior to sintering thereof as it will certainly cause an unacceptable defect such as cracking or flaking in the sintered finished product. Consequently, the debinding process of heating the powder compact for removal of an excess amount of the organic binder therefrom is particularly important for the powder compact formed through the injection molding or the compact containing a great amount of the organic binder. Further, such a debinding or heating process should be performed carefully in order to successfully avoid the occurrence of a defect in the finished product, since wrong or improper control would certainly cause the above unacceptable defects. In fact, when the powder compact is heated too excessively or abruptly, the organic binder within the compact is rapidly decomposed to generate a cracked gas in a high amount per unit of time to thereby increase an internal gas pressure to such an extent as to cause the swelling of the compact, which eventually results in cracking or flaking in the surface of the finished product or even in the fracture thereof.
To avoid the over-heating or abrupt temperature increase, it has been proposed to monitor the furnace temperature and control the heating in accordance with a heating program for reducing the weight of the organic binder at a constant rate in a feedback manner. Although the weight reduction rate of the organic binder can be kept constant with this control, it is not always true that the volume of the cracked gas being generated by decomposition of the organic binder can be increased at a constant rate. Accordingly, the above control alone might in a certain circumstance cause an abrupt increase in the amount of the cracked gas which would result in a defect the finished product. To eliminate the above problem and therefore to make a consistent control during the debinding process, a surrounding condition other than the furnace temperature can be also found important by the reasons as discussed below with reference to FIG. 13A to 13D, which schematically illustrate the debinding process of heating a powder compact 32 formed from a mixture of a sinterable powder and an organic binder within a furnace. As the surrounding temperature of the compact 32 is increased, the organic binder contained in the compact 32 will be decomposed into a cracked gas which spreads in the surface of the compact 32 and subsequently diffuses as vaporized binder into the surrounding space, as shown in FIG. 13 B. As the temperature rises further, the decomposition of the organic binder is enhanced to fill the furnace with the ever generating cracked gas. At this occurrence, the furnace will be entirely saturated with the cracked gas if it is not discharged from the furnace, to thereby inhibit the cracked gas from emerging out of the compact surface, and consequently leave the cracked gas to solidify or become tarry in the surface of the compact, as shown in FIG. 13 C. When the decomposition proceeds further in this condition, the internal gas pressure of the compact 32 is unduly increased to eventually break the compact 32, as shown in FIG. 13D.
In order to prevent such occurrence, the cracked gas is required to be discharged out of the furnace from time to time by feeding a carrier gas in and out of the furnace to expel the cracked gas out of the furnace as being carried therethrough, or by evacuating the furnace with vacuum. The former scheme of refreshing the furnace by the carrier gas is known to be effective for oxide powders but objectionable from an economical standpoint for non-oxide powders as such powder necessitates an expensive inert gas as the carrier gas.
The latter evacuation scheme was utilized in U.S. Pat. No. 2, 939,199 in which the debinding is carried out in vacuum. However, the debinding merely in vacuum cannot successfully control the volume of the cracked gas being generated and will cause the defect in the product. That is, since Boyle's law states that the volume of a given mass of gas is inversely proportional to the pressure at constant temperature; i.e., pV=constant, the volume of the cracked gas increased to a greater extent at a higher level of vacuum. With this result, when the debinding is performed in a predetermined level of vacuum, there is seen a greater expansion of the cracked gas due to the vacuum than at the atmospheric pressure, so that the volume of the gas generated in a temperature range where the decomposition is prevailing is much more increased to thereby leave corresponding voids in the compact which leads to a defect. To avoid this occurrence, it should be therefore required to regulate the vacuum level in accordance with the amount of the cracked gas being generated. Also, as taught in Japanese non-examined patent publication (KOKAI) No. 59-39775, when the debinding is carried out at a high level of vacuum of less than 10.sup.-3 mmHg, the gas will expand to a volume 1,000 times than at atmospheric pressure, which will be also a cause of a defect in the compact and therefore in the finished product.